A method and a device for measuring distances by means of inductive sensors is known from WO 2007/012502 A1. For the purposes of obtaining an electrical measuring signal, for example, as a consequence of a test object approaching an appropriate sensor, several driver coils are operated in clocked manner. The driver coils induce a voltage in a sensor coil at a clock rate of a clock pulse circuit, said signal becoming zero when the circuit is in a steady state. The voltage signals that have been obtained in this way are each divided at the clock rate of the clock pulse circuit into sections which are each associated with a driver coil, these sections being compared with one another so that an amplitude regulating process can be carried out. The difference in value between the voltage signals is used in the course of the amplitude regulating process in order to obtain, from the sections associated with the driver coils, equally large voltage signals that are devoid of clock synchronous components or differences at the inputs of a comparator. When in this steady state, a phase regulating process is additionally started in order to thereby obtain a further component of a measuring signal (e.g. for distinguishing between types of metal).
The principle underlying the amplitude regulating process being used therein is known per se from EP 706 648 B1. Here, light signals between light emitters and light receivers are captured, compensation being made for outside influences such as stray light, temperature or the effects of aging. The light emitters are operated alternately in time division manner using a clock pulse generator. The light that has been amplitude regulated in at least one light path acts on the light receiver, possibly together with the light from a further light emitter such as that from a compensating light source for example, in such a way that a received signal that is devoid of clock synchronous signal components then ensues. The received signal from the light receiver is supplied to a synchronous demodulator which then decomposes the received signal into signal components corresponding to the two sources of light. These are compared with one another in a comparator and a signal corresponding to a zero state is thereby produced. If a signal corresponding to this zero state is not present at the output of the comparator, then the radiating power being supplied to the sources of light is regulated until such time as this state is reached.
Inductive proximity switches are used for example, in order to determine the path or the position in space or the material properties of a test object. In practice thereby, the sensing distance of the proximity sensor is usually very small or different metals such as steel St37, paramagnetic metals such as aluminium or diamagnetic metals such as copper can only be identified with equal sensitivity using special circuit configurations. The solution of WO 2007/012502 A1 could itself contribute to an improvement, but it also requires a phase regulating process in addition to the amplitude regulating process.
DE 33 29 515 C2 discloses an electrical circuit configuration for a magnetic-inductive sensor and a corresponding sensor device for determining the position of a ferromagnetic object. The sensor device comprises several transmitter coils, a receiving device and a clock generator controlling the power supply of the transmitter coils, the recall of the measuring signals from the receiving device and the signal processing components. An electroconductive intermediate mass is arranged between the transmitter coils and the ferromagnetic object. The component recalling the measuring signals is connected to the receiving device only after stabilizing the eddy currents in the intermediate mass.
EP 0 798 502 B1 shows a position/displacement sensor for detecting a valve position. The sensor comprises an evaluation circuit for evaluating the length of a time window effected by a phase shift, an exclusive-or-member and a counter followed by a register, wherein the respective counting results can be read in. An inductive arrangement is provided to detect the valve piston, the arrangement comprising two coils arranged side by side in the direction of movement of the piston. The evaluating device serves for measuring the detuning caused by the change in position of the piston, wherein a phase shift of the AC voltage respectively measured relative to each other is evaluated as detuning
WO 2007/006910 A1 makes known a method for inductive measurement of a relative movement or relative positioning of a first object relative to a second object. For this purpose, two transmitter coils and two receiver coils are provided wherein the transmitter coils are arranged on the first object and the receiver coils are arranged on the second object within a magnetic filed generated by the transmitter coils. The output signal of the receiving coil is used for determining the relative displacement of the objects. The objects to be monitored are parts of a telescope constituted by a plurality of mirror elements. For detecting the position thereof, the common inductivity of the coil arrangement is used and the voltages are measured which are induced in the receiver coils by the transformation effect. For this purpose, the voltage at the transmitting terminals is used, i.e. with a quasi-unlimited impedance.
WO 93/20409 A1 discloses a sensor drive and signal processing. A sensor is primary-side feeded by an oscillating input signal and detects the output signal of the sensor on the secondary side and amplifies it. The secondary side signal processing is digitally conducted. The sensor comprises a coil arrangement consisting of one input coil and two output coils and a core, whose position is detectable.